The field of this invention relates to automatic welding apparatuses, and more particularly to a welding apparatus which is primarily designed to facilitate welding of tubes to the back face of a tube sheet.
A heat exchanger typically consists of a shell in which are located a large number of parallel tubes for carrying cooling liquid. Steam entering the shell condenses on the tubes. At both ends of the heat exchanger, the tubes are held in position by a tube sheet. These spaced-apart tube sheets separate the steam containing shell interior from the remaining portion of the heat exchanger.
In a typical heat exchanger, there may be literally hundreds in number of tubes mounted in a closely spaced arrangement within a tube sheet. It is necessary to insure a leakless joint between each tube and the tube sheet. Rolling or expansion of the tubes into the tube sheet does not provide insurance of a leakless joint. Therefore, it is generally desirable that the tubes be welded to the tube sheet.
It has been found that by manual welding of each of the tubes to the tube sheet, insufficient quality is obtained. The inherent repetitive creating of each annular weld results in the production of an occassional poor quality weld. Even if a single tube is found to leak, that leak must be repaired. Therefore, some means must be utilized to achieve a hundred percent weld quality in every weld.
In order to achieve this hundred percent weld quality, it is common to use an automatic welding apparatus. The automatic welding apparatus is securable in a set position with respect to the welding area. Desirable weld quality is obtained through the use of an inert gas invironment to the welding electrode. The welding current and voltage is monitored through the use of a computer and instantaneous changes will be made if preset desirable perameters are not met during the welding procedure.
Welding of tubes to tube sheets can occur on either face of the tube sheet, which is normally termed either the front face or the back face. Within the front face of the tube sheet, the end of each tube is substantially flush with the front face. Within the back face of the tube sheet, the tubes extend therefrom in a closely spaced array. To achieve welding at the front face, the welding apparatus usually includes a form of a protruding mandrel assembly which extends within each tube to secure in position the welding apparatus in order to achieve the desired welding operation. An example of such a welding apparatus is shown and described within U.S. patent application, Ser. No. 445,090, which was filed Nov. 29, 1982 by the present inventor.
Within the back face, a securing mandrel could not be utilized since the tubes protrude from the back face. However, one way in which the welding assembly can be secured to the back face is for the welding apparatus to be securely mounted onto a particular tube that is to be welded to the back face. The subject matter of the present invention is directed to a welding apparatus which is to be utilized to weld tubes to the back face of a tube sheet.